1. Field of the Invention
The present invention relates generally to communication systems and, more particularly, to providing improved wireless services.
2. Description of Related Art
Global system for mobile communication (GSM) is one of the most widely used wireless access systems in today's fast growing communication systems. GSM provides circuit-switched data services to subscribers, such as mobile telephone or computer users. General Packet Radio Service (GPRS), which is an extension to GSM technology, introduces packet switching to GSM networks. GPRS uses a packet-based wireless communication technology to transfer high and low speed data and signaling in an efficient manner. GPRS optimizes the use of network and radio resources, thus enabling the cost effective and efficient use of GSM network resources for packet mode data applications. FIG. 1 shows a conventional GSM/GPRS network architecture.
As shown in FIG. 1, GSM/GPRS network 100 includes a GSM core network 101 and a GPRS network 130. GSM core network 101 includes a Mobile Station (MS) 102, at least one Base Transceiver Station (BTS) 104, and a Base Station Controller (BSC) 106. MS 102 is physical equipment or Mobile Equipment (ME), such as a mobile phone or a laptop computer that is used by mobile subscribers, with a Subscriber Identity Module (SIM). The SIM includes an International Mobile Subscriber Identity (IMSI), which is a unique identifier of a subscriber. BTS 104 is physical equipment, such as a radio tower, that enables a radio interface to communicate with the MS. Each BTS may serve more than one MS. BSC 106 manages radio resources, including the BTS. The BSC may be connected to several BTSs. The BSC and BTS components, in combination, are generally referred to as a Base Station System (BSS).
GSM core network 101 also includes a Mobile Switching Center (MSC) 108, a Gateway Mobile Switching Center (GMSC) 110, a Home Location Register (HLR) 112, a Visitor Location Register (VLR) 114, an Authentication Center (AuC) 116, and an Equipment Identity Register (EIR) 118. MSC 108 performs a switching function for the network. The MSC also performs other functions, such as registration, authentication, location updating, handovers, and call routing. GMSC 110 provides a gateway between the GSM network and other networks, such as an Integrated Services Digital Network (ISDN) or Public Switched Telephone Networks (PSTNs) 120. In other words, the GMSC provides interworking functionality with external networks.
HLR 112 is a database that contains administrative information regarding each subscriber registered in a corresponding GSM network. The HLR also contains the current location of each MS. VLR 114 is a database that contains selected administrative information from the HLR. The VLR contains information necessary for call control and provision of subscribed services for each MS currently located in a geographical area controlled by the VLR. The HLR and the VLR, together with the MSC, provide the call routing and roaming capabilities of GSM. AuC 116 provides the parameters needed for authentication and encryption functions. Such parameters allow verification of a subscriber's identity. EIR 118 stores security-sensitive information about the mobile equipment.
To gain access to GSM services, such as speech, data, and short message service (SMS), the MS first registers with the network to indicate its current location by performing a location update and IMSI attach procedure. The MS sends a location update message including its current location information to the MSC/VLR, via the BTS and the BSC. The location information is then sent to the MS's HLR. The HLR is updated with the location information received from the MSC/VLR. The location update is also performed when the MS moves to a new location area. Typically, the location update is periodically performed to update the database as location updating events occur.
GPRS network 130 is logically implemented on the GSM core network architecture by introducing two packet-switching network nodes, a Serving GPRS support node (SGSN) 132 and a Gateway GPRS support node (GGSN) 134. SGSN 132 is at the same hierarchical level as the MSC in the GSM network. The SGSN controls the connection between the GPRS network and the MS. The SGSN also keeps track of individual MS's locations and manages security functions and access controls. GGSN 134 provides a gateway between the GPRS network and a public packet data network (PDN) or other GPRS networks 136. That is, the GGSN provides interworking functionality with external packet data networks, and sets up a logical link to the MS through the SGSN. When packet-switched data leaves the GPRS network, it is transferred to an external TCP-IP network 136, such as an X.25 network or the Internet. In order to access GPRS services, the MS first attaches itself to the GPRS network by performing an attach procedure. The MS then activates a packet data protocol (PDP) context, thus activating a packet communication session between the MS, the SGSN, and the GGSN.
In a GSM/GPRS network, GPRS services and GSM services can be used in parallel. The MS can operate in one of three classes: class A, class B, and class C. A class A MS can attach to the network for both GPRS services and GSM services simultaneously. Class A MS also supports simultaneous operation of GPRS services and GSM services. For example, class A mobiles can receive GSM voice/data/SMS calls and GPRS data calls at the same time.
A class B MS also can attach to the network for both GPRS services and GSM services simultaneously. However, a class B MS does not support simultaneous operation of GPRS services and GSM services. That is, a class B MS can only use one of the two services at a given time.
A class C MS can attach for only one of the GPRS services and GSM services at a time. Simultaneous attachment and operation of GPRS services and GSM services is not possible with a class C MS.
Currently, a GPRS network can be designed to operate in three network operation modes NOM1, NOM2, and NOM3). A network operation mode of a GPRS network is indicated by a parameter in system information messages transmitted within a cell. The system information messages dictates a MS where to listen for paging messages and how to signal towards the network. The network operation mode represents the capabilities of the GPRS network. In NOM1 network, a MS can receive pages from a circuit switched domain (voice call) when engaged in a data call. The MS can suspend the data call or take both simultaneously, depending on the ability of the MS. In NOM2 network, a MS may not receive pages from a circuit switched domain when engaged in a data call, since the MS is receiving data and is not listening to a paging channel. In NOM3 network, a MS can monitor pages from a circuit switched network while receiving data and vice versa.
In NOM 1, a MS performs a combined attach procedure. The combined attach is similar to the GSM registration process where the MS performs a location update and IMSI (GPRS) attach. The MS initiates the combined attach procedure by sending a request to the network for both location update for circuit switched operation and GPRS attach for packet switched services. FIG. 2 shows a combined attach procedure in accordance with existing GSM/GPRS standards. The MS first sends an ATTACH REQUEST message including the IMSI to the SGSN, and starts a first timer set to expire after 15 seconds. The SGSN receives the message from the MS via a Gb interface (see FIG. 1). The SGSN may authenticate the MS. The SGSN then sends a LOCATION UPDATE REQUEST message to the MSC/VLR, via a Gs interface, for the location update procedure. The location update procedure establishes SGSN-VLR association and allows the VLR to maintain current location information of the MS. Once the location update procedure is completed, a LOCATION UPDATE ACCEPT message is sent to the SGSN. If the combined attach is accepted by the network, the SGSN sends an ATTACH ACCEPT message including a location area identification (LAI) to the MS. Upon receiving the ATTACH ACCEPT message, the MS stores the LAI and stops the first timer. The MS then sends an ATTACH COMPLETE message to the SGSN.
In accordance with existing standards, if the combined attach is not accepted by the network by the first expiry of the first timer, the MS resets and restarts the first timer, and the MS retransmits the ATTACH REQUEST message. This retransmission is repeated four times for a total of about one minute of attempts. On the fifth expiry of the first timer, the combined attach procedure is aborted and an attach attempt counter is incremented. If the attach attempt counter is less than five, a second timer, set to expire after 15 seconds, is started. Upon the expiry of the second timer, the combined attach procedure is repeated. When the attach attempt counter reaches five, the MS-aborts the combined attach procedure and starts a third timer. The MS then receives an ATTACH REJECT message, which includes a reject cause, from the network. If the rejection cause included in the ATTACH REJECT message indicates that the GPRS is the cause of the rejection, the MS attempts to gain access only to the GSM services by performing the location update procedure. The MS sends a location update message directly to the MSC/VLR.
Thus, under present GSM/GPRS standards, when a combined attach is not accepted by the network, a MS attempts the combined attach for about 25 times (i.e., for more than seven minutes) before the MS attempts to gain access only to GSM services. During this time period, the MS (i.e., a subscriber) is unable to receive any GSM services, even though the GSM network may be available to the MS.